Chickens which were given cyclophosphamide during the first three to five days of life showed a marked depression in their humoral antibody response to sheep erythrocytes and Brucella antigen. Their ability to develop tuberculin sensitivity and their immune response to live Salmonella gallinarum vaccine was, however, unimpaired. The administration of methylprednisoline acetate, hydrocortisone acetate, azathioprim and 6-mercaptopurine to either six or 10 to 12-week-old chickens neither selectively depressed the humoral or cellular immune response nor affected the immune response to live S. gallinarum vaccine. Similarly, the immune response could not be depressed by thymectomy or antilymphocyte globulin. These results indicate that immunity to S. gallinarum after administration of a live avirulent vaccine is not dependent on a humoral immune mechanism but probably primarily on cellular immunity, although this could not be proven.